Re: [PATCH 2/3] migration/postcopy: unsentmap is not necessary for postcopy

[Dr. David Alan Gilbert]

* Wei Yang (address@hidden) wrote:
> Commit f3f491fcd6dd594ba695 ('Postcopy: Maintain unsentmap') introduced
> unsentmap to track not yet sent pages.
> 
> This is not necessary since:
> 
>     * unsentmap is a sub-set of bmap before postcopy start
>     * unsentmap is the summation of bmap and unsentmap after canonicalizing
> 
> This patch just removes it.
> 
> Signed-off-by: Wei Yang <address@hidden>

Yes, that's simpler


Reviewed-by: Dr. David Alan Gilbert <address@hidden>

> ---
>  include/exec/ram_addr.h |  6 ----
>  migration/ram.c         | 80 ++++++++---------------------------------
>  2 files changed, 14 insertions(+), 72 deletions(-)
> 
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index a327a80cfe..e96e621de5 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -44,12 +44,6 @@ struct RAMBlock {
>      size_t page_size;
>      /* dirty bitmap used during migration */
>      unsigned long *bmap;
> -    /* bitmap of pages that haven't been sent even once
> -     * only maintained and used in postcopy at the moment
> -     * where it's used to send the dirtymap at the start
> -     * of the postcopy phase
> -     */
> -    unsigned long *unsentmap;
>      /* bitmap of already received pages in postcopy */
>      unsigned long *receivedmap;
>  
> diff --git a/migration/ram.c b/migration/ram.c
> index 075ddc468c..066eb4755f 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2348,7 +2348,7 @@ static bool get_queued_page(RAMState *rs, 
> PageSearchStatus *pss)
>              dirty = test_bit(page, block->bmap);
>              if (!dirty) {
>                  trace_get_queued_page_not_dirty(block->idstr, 
> (uint64_t)offset,
> -                       page, test_bit(page, block->unsentmap));
> +                       page, test_bit(page, block->bmap));

We could actually lose that last trace parameter.

Dave

>              } else {
>                  trace_get_queued_page(block->idstr, (uint64_t)offset, page);
>              }
> @@ -2619,10 +2619,6 @@ static int ram_save_host_page(RAMState *rs, 
> PageSearchStatus *pss,
>          }
>  
>          pages += tmppages;
> -        if (pss->block->unsentmap) {
> -            clear_bit(pss->page, pss->block->unsentmap);
> -        }
> -
>          pss->page++;
>      } while ((pss->page & (pagesize_bits - 1)) &&
>               offset_in_ramblock(pss->block, pss->page << TARGET_PAGE_BITS));
> @@ -2776,8 +2772,6 @@ static void ram_save_cleanup(void *opaque)
>          block->clear_bmap = NULL;
>          g_free(block->bmap);
>          block->bmap = NULL;
> -        g_free(block->unsentmap);
> -        block->unsentmap = NULL;
>      }
>  
>      xbzrle_cleanup();
> @@ -2857,8 +2851,6 @@ void 
> ram_postcopy_migrated_memory_release(MigrationState *ms)
>   * Returns zero on success
>   *
>   * Callback from postcopy_each_ram_send_discard for each RAMBlock
> - * Note: At this point the 'unsentmap' is the processed bitmap combined
> - *       with the dirtymap; so a '1' means it's either dirty or unsent.
>   *
>   * @ms: current migration state
>   * @block: RAMBlock to discard
> @@ -2867,17 +2859,17 @@ static int 
> postcopy_send_discard_bm_ram(MigrationState *ms, RAMBlock *block)
>  {
>      unsigned long end = block->used_length >> TARGET_PAGE_BITS;
>      unsigned long current;
> -    unsigned long *unsentmap = block->unsentmap;
> +    unsigned long *bitmap = block->bmap;
>  
>      for (current = 0; current < end; ) {
> -        unsigned long one = find_next_bit(unsentmap, end, current);
> +        unsigned long one = find_next_bit(bitmap, end, current);
>          unsigned long zero, discard_length;
>  
>          if (one >= end) {
>              break;
>          }
>  
> -        zero = find_next_zero_bit(unsentmap, end, one + 1);
> +        zero = find_next_zero_bit(bitmap, end, one + 1);
>  
>          if (zero >= end) {
>              discard_length = end - one;
> @@ -2938,16 +2930,12 @@ static int 
> postcopy_each_ram_send_discard(MigrationState *ms)
>   * clean, not a mix.  This function canonicalizes the bitmaps.
>   *
>   * @ms: current migration state
> - * @unsent_pass: if true we need to canonicalize partially unsent host pages
> - *               otherwise we need to canonicalize partially dirty host pages
>   * @block: block that contains the page we want to canonicalize
>   */
> -static void postcopy_chunk_hostpages_pass(MigrationState *ms, bool 
> unsent_pass,
> -                                          RAMBlock *block)
> +static void postcopy_chunk_hostpages_pass(MigrationState *ms, RAMBlock 
> *block)
>  {
>      RAMState *rs = ram_state;
>      unsigned long *bitmap = block->bmap;
> -    unsigned long *unsentmap = block->unsentmap;
>      unsigned int host_ratio = block->page_size / TARGET_PAGE_SIZE;
>      unsigned long pages = block->used_length >> TARGET_PAGE_BITS;
>      unsigned long run_start;
> @@ -2957,13 +2945,8 @@ static void 
> postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
>          return;
>      }
>  
> -    if (unsent_pass) {
> -        /* Find a sent page */
> -        run_start = find_next_zero_bit(unsentmap, pages, 0);
> -    } else {
> -        /* Find a dirty page */
> -        run_start = find_next_bit(bitmap, pages, 0);
> -    }
> +    /* Find a dirty page */
> +    run_start = find_next_bit(bitmap, pages, 0);
>  
>      while (run_start < pages) {
>  
> @@ -2973,11 +2956,7 @@ static void 
> postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
>           */
>          if (QEMU_IS_ALIGNED(run_start, host_ratio)) {
>              /* Find the end of this run */
> -            if (unsent_pass) {
> -                run_start = find_next_bit(unsentmap, pages, run_start + 1);
> -            } else {
> -                run_start = find_next_zero_bit(bitmap, pages, run_start + 1);
> -            }
> +            run_start = find_next_zero_bit(bitmap, pages, run_start + 1);
>              /*
>               * If the end isn't at the start of a host page, then the
>               * run doesn't finish at the end of a host page
> @@ -2994,9 +2973,6 @@ static void 
> postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
>              /* Clean up the bitmap */
>              for (page = fixup_start_addr;
>                   page < fixup_start_addr + host_ratio; page++) {
> -                /* All pages in this host page are now not sent */
> -                set_bit(page, unsentmap);
> -
>                  /*
>                   * Remark them as dirty, updating the count for any pages
>                   * that weren't previously dirty.
> @@ -3005,13 +2981,8 @@ static void 
> postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
>              }
>          }
>  
> -        if (unsent_pass) {
> -            /* Find the next sent page for the next iteration */
> -            run_start = find_next_zero_bit(unsentmap, pages, run_start);
> -        } else {
> -            /* Find the next dirty page for the next iteration */
> -            run_start = find_next_bit(bitmap, pages, run_start);
> -        }
> +        /* Find the next dirty page for the next iteration */
> +        run_start = find_next_bit(bitmap, pages, run_start);
>      }
>  }
>  
> @@ -3033,13 +3004,10 @@ static int postcopy_chunk_hostpages(MigrationState 
> *ms, RAMBlock *block)
>  {
>      postcopy_discard_send_init(ms, block->idstr);
>  
> -    /* First pass: Discard all partially sent host pages */
> -    postcopy_chunk_hostpages_pass(ms, true, block);
>      /*
> -     * Second pass: Ensure that all partially dirty host pages are made
> -     * fully dirty.
> +     * Ensure that all partially dirty host pages are made fully dirty.
>       */
> -    postcopy_chunk_hostpages_pass(ms, false, block);
> +    postcopy_chunk_hostpages_pass(ms, block);
>  
>      postcopy_discard_send_finish(ms);
>      return 0;
> @@ -3077,19 +3045,6 @@ int ram_postcopy_send_discard_bitmap(MigrationState 
> *ms)
>      rs->last_page = 0;
>  
>      RAMBLOCK_FOREACH_NOT_IGNORED(block) {
> -        unsigned long pages = block->used_length >> TARGET_PAGE_BITS;
> -        unsigned long *bitmap = block->bmap;
> -        unsigned long *unsentmap = block->unsentmap;
> -
> -        if (!unsentmap) {
> -            /* We don't have a safe way to resize the sentmap, so
> -             * if the bitmap was resized it will be NULL at this
> -             * point.
> -             */
> -            error_report("migration ram resized during precopy phase");
> -            rcu_read_unlock();
> -            return -EINVAL;
> -        }
>          /* Deal with TPS != HPS and huge pages */
>          ret = postcopy_chunk_hostpages(ms, block);
>          if (ret) {
> @@ -3097,12 +3052,9 @@ int ram_postcopy_send_discard_bitmap(MigrationState 
> *ms)
>              return ret;
>          }
>  
> -        /*
> -         * Update the unsentmap to be unsentmap = unsentmap | dirty
> -         */
> -        bitmap_or(unsentmap, unsentmap, bitmap, pages);
>  #ifdef DEBUG_POSTCOPY
> -        ram_debug_dump_bitmap(unsentmap, true, pages);
> +        ram_debug_dump_bitmap(block->bmap, true,
> +                              block->used_length >> TARGET_PAGE_BITS);
>  #endif
>      }
>      trace_ram_postcopy_send_discard_bitmap();
> @@ -3270,10 +3222,6 @@ static void ram_list_init_bitmaps(void)
>              bitmap_set(block->bmap, 0, pages);
>              block->clear_bmap_shift = shift;
>              block->clear_bmap = bitmap_new(clear_bmap_size(pages, shift));
> -            if (migrate_postcopy_ram()) {
> -                block->unsentmap = bitmap_new(pages);
> -                bitmap_set(block->unsentmap, 0, pages);
> -            }
>          }
>      }
>  }
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK